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Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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156 FXUS63 KDMX 121733 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1233 PM CDT Mon Aug 12 2024 ...Updated for the 18z Aviation Discussion... .KEY MESSAGES... - Showers and a few storms through mid-morning, mainly central and south. Lingering isolated showers & storms into the afternoon. Severe weather not expected. - Showers and storms likely midweek with potential for locally heavy rains. River flooding not anticipated. - Return of more seasonal warmth and humid conditions by midweek && .DISCUSSION... Issued at 249 AM CDT Mon Aug 12 2024 After the end or reduction of this morning`s convection our pattern will remain progressive highlighted by upper level ridging through the MO Valley by Tuesday night, lowering heights by midweek as a central/northern Plains upper trough matures into the MS Valley by Friday morning, and then northwest flow by the weekend as upper level ridging builds through the Rockies. Currently PV anomalies near Omaha and more notably over SD continue to provide both kinematic and thermodynamic support for weak convection this morning. Much of the lift seems to be driven by warm/theta-e advection associated with an H85/H7 front and further aided by a nose of moisture into that layer`s baroclinic zone. RAP soundings suggest the column is fairly saturated right now, limiting elevated/MUCAPEs to no more than 500-750 J/kg rooted 1-2km through this event. Effective shear does jump up to ~40kts at times, but 00z HREF updraft strengths suggest instability will be insufficient to realize that shear and limit any severe potential. 00Z HREF and 06z HRRR runs also continue to highlight more appreciable rainfall accumulations farther south into MO along the better 0-2km moisture convergence. Much of the precip should move to the east by mid morning, however isolated showers and a few storms may linger into the afternoon in weak forcing and deeper moisture ahead of the approaching eastern SD PV anomaly. Although much of the precip should end by 00z, fog may develop tonight in lingering low level moisture and weak southeast surface flow. After a break Tuesday, our attention will then turn to more precip potential from late Tuesday night through Thursday. While model consensus for the middle of next week seemed to be improving at this time yesterday, that does not appear to be the case this morning with models again struggling on short wave timing, strength, location, and of course similar questions with associated surface features. However regardless of solution there appears to be an active period with the most likely scenario suggesting a theta-e advection driven MCS late Tuesday night into Wednesday, and then a period more focused with a surface wave sometime Thursday. This will also coincide with a more summer-like airmass, with humid conditions as dewpoints rise well through the 60s, and to 70F+ at times. Temps appear a bit more tempered versus this time yesterday however, which may limit instability somewhat. This and weaker flow aloft may keep any severe weather potential relatively low, although not ruled out. Locally heavy rains would appear to be a somewhat greater concern. 11/12z NAEFS and EC ensemble data have been consistent depicting very anomalous low level specific humidities and precipitable water values nearby Tue night through Thu with percentiles well through the upper 90s. 00Z GFS depicts precipitable waters ~2" with 4-4.5km warm cloud depths, all suggesting efficient rainfall. 01Z NBM 50th percentile 48hr QMD QPF also suggests widespread ~1.75" possible across the Mid MS Valley Wed and Thu, which is notable for parameterized convective precip. The max location is not in our forecast area, but with model uncertainties with regard to details it is more of a flag that locally heavy, efficient rainfall will be possible in the vicinity. While rainfall may be appreciable, recent dry conditions have tempered any river flood concerns with only within bank rises suggested based on HEFS median precip, and deterministic 120hr QPF. Ponding/standing water and possibly a few flash flooding events would be more of a concern. Most solutions show the parent upper low and associated surface reflection to the east by Friday leaving us in northwest flow aloft through the weekend and remainder of the period. The pattern should mainly be dry with seasonal temps and humidity, although spotty precip cannot be ruled out depending on the strength of waves in the northern stream and northwest flow. && .AVIATION /18Z TAFS THROUGH 18Z TUESDAY/... Issued at 1233 PM CDT Mon Aug 12 2024 Lingering spotty showers across central Iowa this afternoon with impacts at KDSM, including MVFR ceilings. Overnight fog is expected with MVFR to IFR visibility and ceiling reductions across most sites. While visibility is expected to improve after 12-13z at most sites, lingering low stratus will continue to impact sites through midday Tuesday. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Small AVIATION...Hagenhoff